Butadiene is an important basic chemical and is used, for example, for preparing synthetic rubbers (butadiene homopolymers, styrene-butadiene rubber or nitrile rubber) or for preparing thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene can also be converted into sulfolane, chloroprene and 1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile). Furthermore, vinylcyclohexene can be produced by dimerization of butadiene and can be dehydrogenated to styrene.
Butadiene can be prepared by thermal dissociation (steam cracking) of saturated hydrocarbons, with naphtha usually being used as raw material. In the steam cracking of naphtha, a hydrocarbon mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butanes, butenes, butadiene, butynes, methylallene, C5-hydrocarbons and higher hydrocarbons is obtained.
Butadiene can also be obtained by oxidative dehydrogenation of n-butenes (1-butene and/or 2-butene). Any desired mixture comprising n-butenes can be utilized as starting gas mixture for the oxidative dehydrogenation of the n-butenes to butadiene. For example, it is possible to use a fraction which comprises n-butenes (1-butene and/or 2-butene) as main constituent and has been obtained from the C4 fraction from a naphtha cracker by removal of butadiene and isobutene. Furthermore, it is also possible to use gas mixtures which comprise 1-butene, cis-2-butene, trans-2-butene or mixtures thereof and have been obtained by dimerization of ethylene as starting gas. Gas mixtures which comprise n-butenes and have been obtained by fluid catalytic cracking (FCC) can also be used as starting gas.
Gas mixtures which comprise n-butenes and are used as starting gas in the oxidative dehydrogenation of n-butenes to butadiene can also be produced by nonoxidative dehydrogenation of n-butane-comprising gas mixtures. WO2005/063658 discloses a process for preparing butadiene from n-butane, which comprises the steps    a) provision of a feed gas stream a comprising n-butane;    b) introduction of the feed gas stream a comprising n-butane into at least one first dehydrogenation zone and nonoxidative catalytic dehydrogenation of n-butane, giving a product gas stream b comprising n-butane, 1-butene, 2-butene, butadiene, hydrogen, low-boiling secondary constituents and possibly water vapor;    c) introduction of the product gas stream b from the nonoxidative catalytic dehydrogenation and an oxygen-comprising gas into at least one second dehydrogenation zone and oxidative dehydrogenation of 1-butene and 2-butene, giving a product gas stream c which comprises n-butane, 2-butene, butadiene, hydrogen, low-boiling secondary constituents and water vapor and has a higher content of butadiene than the product gas stream b;    d) removal of hydrogen, the low-boiling secondary constituents and water vapor to give a C4 product gas stream d which consists essentially of n-butane, 2-butene and butadiene;    e) separation of the C4 product gas stream d into a recycle stream e1 consisting essentially of n-butane and 2-butene and a stream e2 consisting essentially of butadiene by extractive distillation and recirculation of the stream e1 to the first dehydrogenation zone.
This process has particularly effective utilization of the raw materials. Thus, losses of the raw material n-butane are minimized by recirculation of unreacted n-butane to the dehydrogenation. A high butadiene yield is achieved by the coupling of nonoxidative catalytic dehydrogenation and oxidative dehydrogenation. Compared to the production of butadiene by cracking, the process has a high selectivity. No coproducts are obtained. The complicated separation of butadiene from the product gas mixture from the cracking process is dispensed with.
The residual oxygen can be a problem since it can bring about butadiene peroxide formation in downstream process steps and can act as initiator for polymerization reactions. This risk exists particularly in regions in which butadiene is present in high concentrations and as pure material, since it is known that unstabilized 1,3-butadiene can form hazardous butadiene peroxides in the presence of oxygen. In the above-described process, this risk is present particularly in step e) in which butadiene is separated from the remaining C4-hydrocarbons. These risks are discussed, for example, by D. S. Alexander (Industrial and Engineering Chemistry 1959, 51, 733-738). These peroxides are viscous liquids. Their density is greater than that of butadiene. In addition, since they are only sparingly soluble in liquid 1,3-butadiene, they settle out on the bottom of storage vessels. Despite their relatively low chemical reactivity, the peroxides are very unstable compounds which can decompose spontaneously at temperatures in the range from 85 to 110° C. A particular hazard is the high shock sensitivity of the peroxides which explode with the brisance of an explosive. The risk of polymer formation is present, in particular, in the isolation of butadiene by distillation and can there lead to deposits of polymers (formation of “popcorn”) in the columns.
WO2006/05969 describes a process for oxidative dehydrogenation in which removal of oxygen is carried out immediately after the oxidative dehydrogenation by means of a catalytic combustion step. In the catalytic combustion step, the oxygen is reacted with hydrogen added in this step in the presence of a catalyst.
This can reduce the oxygen content to small traces. Disadvantages of the catalytic combustion step are the consumption of hydrogen and also secondary reactions in which part of the 1,3-butadiene is burnt in the presence of oxygen and hydrogenated by hydrogen to form butene and butane.